1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive employing active braking using inductive sense.
2. Description of the Prior Art
FIG. 1 shows a prior art disk drive 2 comprising a disk 4 having a plurality of tracks, a head 6, a voice coil motor 8 for actuating the head 6 radially over the disk 4, and an interface 10 for receiving a primary supply voltage 12 and a secondary supply voltage 14 from a host computer. The disk drive 2 further comprises a multi-phase spindle motor 16 for rotating the disk 4, wherein the multi-phase spindle motor 16 comprises a plurality of windings (e.g., φA, φB, φC) having a first end and a second end, wherein the second ends are connected together at a center tap 18. A spindle driver 20, responsive to the primary supply voltage 12, commutates the windings over commutation intervals. The spindle motor 16 is shown as comprising three windings (φA, φB, φC) corresponding to three phases. However, any suitable number of windings may be employed to implement any suitable multi-phase spindle motor. Further, any suitable commutation sequence may be employed to commutate the windings. For example, the commutation logic 22 may control switches 23 to commutate the windings of the spindle motor 16 in a two-phase, three-phase, or hybrid two-phase/three-phase commutation sequence.
Disk control circuitry 24 communicates with the host computer over interface 11 and executes various operations (e.g., servo control, read/write channel, etc.) to perform read and write commands. The disk control circuitry 24 generates a control signal 26 and a pulse width modulated (PWM) signal 28 applied to the spindle driver 20. The control signal 26 comprises control information (such as a starting state and a commutation clock), and the PWM signal 28 is used to modulate the driving current in the windings. The windings are connected to a back EMF detector 30 which detects threshold crossings (e.g., zero crossings) in the back EMF voltage generated by the windings with respect to the center tap 18. Since the back EMF voltage is distorted when current is flowing, the spindle driver 20 supplies a control signal 32 to the back EMF detector 30 identifying the “open” winding generating a valid back EMF signal. At each back EMF threshold crossing the back EMF detector 30 toggles a signal to generate a square wave signal 34. The frequency of the back EMF threshold crossings and thus the frequency of the square wave signal 34 represent the speed of the spindle motor 16. The disk control circuitry 24 evaluates the square wave signal 34 and adjusts the PWM signal 28 in order to control the speed of the spindle motor 16.
The disk drive 2 of FIG. 1 further comprises a voice coil motor (VCM) driver 36 responsive to the primary and secondary supply voltages 12 and 14. The VCM driver 36 applies the primary supply voltage 12 to the voice coil motor 8 through driver 37 either in a linear power amplifier mode or in a modulated sequence (e.g., PWM) to control the speed of the voice coil motor 8 while actuating the head 6 radially over the disk 4. The secondary supply voltage 14 powers circuitry within the VCM driver 36 as well as other circuitry within the disk drive 2, such as the spindle driver 20 and disk control circuitry 24. In one embodiment, the primary supply voltage 12 comprises twelve volts and the secondary supply voltage 14 comprises five volts. In an alternative embodiment, the disk drive 2 receives a single supply voltage (e.g., five volts) for driving the VCM 8 and spindle motor 16 and for powering circuitry in the disk drive 2.
Under certain circumstances, the disk drive 2 parks the head 6 and spins down the disk 4, for example in portable applications in order to conserve battery life while the disk drive is idle. If the head 6 is parked on the disk 4 in a landing zone (e.g., at an inner diameter of the disk), the air bearing that supports the head 6 eventually dissipates until the head 6 contacts the disk 4 as the disk 4 spins to a stop. In order to minimize head wear, it is important to stop the disk 4 as soon as possible after the head 6 contacts the disk 4. Prior art spin-down techniques employ active braking wherein the current driving the spindle motor 16 is reversed while commutating the windings in response to the back EMF threshold crossings 34, thereby applying a braking torque to the spindle motor 16. When the velocity of the spindle motor 16 decreases to a point that the back EMF threshold crossings 34 are no longer reliable, prior art disk drives employ dynamic braking wherein the windings of the spindle motor 16 are simply shorted to brake the spindle motor 16 until the disk 4 stops spinning. However, a significant amount of head wear can still occur while dynamic braking at low RPMs since the braking power decays as the current in the windings decays.
There is, therefore, a need to reduce the spin-down time in a disk drive in order to reduce head wear.